There are today different concepts for distributing base station functionality onto different nodes into a so called distributed base station system. Basic purposes for distributing base station functionality are to improve radio coverage and to increase throughput to pieces of User Equipment, UEs. Base station functionality is typically distributed onto one or more base units, one or more intermediate units and a plurality of remote units. Typically, the remote unit has little functionality and can be made cheap and small, and the more intelligent functionality of a base station is moved up in the system. Thereby, it may be cost-efficient for an operator to have many remote units close to the antenna with a small geographical coverage area each, and to connect the remote units via e.g. a cable to an intermediate unit. Further, a distributed base station may be cost-efficient as the base units, which are processing-heavy are centralized and can therefore share electrical power, cooling systems, processors etc. Also, to centralize base stations allows for co-ordination of traffic, e.g. mobility management, over a large coverage area.
One type of distributed base station is a regular base station connected to a distributed antenna system, DAS. In a DAS, a plurality of remote antennas are physically connected to an intermediate controller which in its turn is connected to a base station. The base station may, except for communicating with UEs via the intermediate controller and the antennas, also have its own antennas serving UEs directly. In an active DAS, the nodes, i.e. intermediate controller and/or remote antennas contain means for amplifying the signal, and in some cases also for translating the signal to a different frequency. The active DAS is commonly used for different kinds of radio deployments such as indoor enterprise environments, shopping malls, airports, railway stations, arenas and even for outdoor light pole micro cell deployments. The active DAS supports multi-operator operation, base stations from different vendors, multi-band operation and multi-access operation. The active DAS can also be deployed over fiber or over copper cable, e.g. twisted pair; CAT5, 6 or 7 or coaxial cable.
FIG. 1 shows an active DAS 15 according to prior art. The active DAS comprises an intermediate controller, also called a head-end unit 30 connected to one or more remote units 20. One or more base stations 40, 50 are typically connected to the head-end 30, which may be a common head-end, with analog radio frequency interfaces, the antenna interface. A DAS vendor interface may be used for communication to the remote unit 20. The interface is potentially proprietary. The remote unit 20 is either connected towards local antennas 10 which might be integrated into the unit or, as more common, to a passive DAS portion comprising a tree-coax cabling network for a local distribution of passive DAS antennas. The remote unit 20 can contain several RF modules, typically one module per frequency band, and per operator and possibly per radio frequency band, feeding the analog RF carrier signals to the local antennas or the passive DAS portion for further wireless distribution to UEs 40. For the interface 25 between the head-end 30 and the remote unit 20, optical fiber or CAT cabling can be used. The radio carriers are transported over the interface 25 either by analog sub-carrier multiplexing or by digital packet transmission. Such cabling limits however the available capacity between head-end and remote unit and digital signal processing is typically used to configure the carriers/sectors into the interface. The head-end and remote unit are often equipped with specific hardware both for the radio carrier frequency(ies) and for the cable type used.
Another type of distributed base station is called a remote radio head system RRHS. The RRHS can be described as spatially separated transceivers connected to a radio unit via corresponding antenna ports. The RRHS is in one perspective a complete radio base station with split architecture, but can also be seen as a type of active DAS, possibly with integrated antenna in the remote unit. The RRHS enables operators to utilize e.g. LAN cables like CAT6/7 for indoor radio deployments. This system improves over older distributed antenna systems by providing streamlined installation procedures, low cost, energy efficiency and higher capacity due to native support of MIMO and multi-band. The RRHS 55 is a distributed base station system wherein the base station functionality is separated in different nodes, which are shown in FIG. 2. A baseband unit, BBU 60, in which signal treatment in the baseband frequency area is performed, is connected to an intermediate radio unit, IRU 70, which is arranged to receive (in the downlink direction) the baseband signals from one or more BBUs, convert them to an intermediate frequency and distribute the signals over a dedicated cable 75 to a destined radio head, RH 80. There may be a plurality of RHs connected via dedicated cables to the IRU. The RH 80 then up-converts the received intermediate frequency signal to a radio frequency for radio transmission from an antenna 90 of the RH towards UEs 40 being in radio connection with the RH. The IRU 70 does not have an analog radio interface towards the BBU, like the typical active DAS has towards the base station. Instead an electrical, or optical, CPRI interface is used to connect to the BBU, which saves hardware both in the base station and in the IRU. Another difference, compared to the typical active DAS, is that the proprietary interface over cable 75 is analog intermediate frequency IF rather than digital.
The CPRI is not yet standardized on all protocol levels but may be in the future. Alternatively, other optical/digital interfaces may be standardized. This would enable multi-vendor digital connection between BBUs/base stations and the IRU/head end. An analog, e.g. RF, interface between the BBU/base station and the IRU/head end would similarly enable multi-vendor and multi-operator services.
The main objective with multi-operator solutions is to share infrastructure, in terms of cabling, radios, etc. Depending on solution, more or less equipment can be shared. The most common solution today is passive DAS, where only passive infrastructure is shared, i.e. cables, splitters, antennas, especially multi-band antennas, and then the active radio equipment is specific per operator. Several methods for multi-operator solutions have been discussed, involving both licensed and un-licensed spectrum, for example (the list is not conclusive): Operators using separate, e.g. own or licensed, spectrum within the same frequency band; Operators using separate e.g. own or licensed, spectrum within different frequency bands; Operators using combined e.g. own or licensed, spectrum within the same frequency band; Operators using separate e.g. own or licensed, spectrum within the same frequency band, in combination with shared unlicensed spectrum, called License Assisted Access, LAA; Operators using shared, licensed or unlicensed, spectrum within one or multiple frequency bands.
License Assisted Access, LAA. 3GPP standardization work has started to support aggregation of licensed and unlicensed spectrum with LTE. A licensed carrier is used as anchor for mobility and control information, and an unlicensed carrier is used for increasing capacity and peak rates through carrier aggregation. In an indoor multi-operator scenario the indoor system must support at least one licensed carrier for each operator, but this carrier may be of a very low bandwidth, LTE supports carriers as narrow as 1.4 MHz or possibly even narrower, if aggregation with one or several unlicensed carriers is possible. The unlicensed carriers may be shared dynamically between the operators depending on capacity demands and/or contractual agreements Service Level Agreements, SLAs.
Since the RRHS is today really an eNodeB, multi-operator sharing in the sense that operators share the active equipment to transmit on their respective licensed carriers is not possible in today's RRHS. However, it is expected that the interface between the BBU and the IRU will be evolved such that it can enable multi-operator support. In the multi-operator case, there is often a need to support indoor coverage for all operators. When deploying many operators and carriers with sufficient radio coverage, the potential air-interface capacity is often superfluous. The cable, used between the IRU/head-end and the radio head/remote unit, has a limited bandwidth which can be the bottle neck for supporting several operators if each operator needs a dedicated carrier.